In a silver halide photographic element, a color image is formed when the element is exposed to light and then subjected to color development with a primary aromatic amine developer. Color development results in imagewise reduction of silver halide and production of oxidized developer. Oxidized developer reacts with one or more incorporated dye-forming couplers to form an imagewise distribution of dye.
The dyes that are formed by any color coupler during processing have a tendency to fade over time as a result of exposure to light, heat and humidity. As all three image dyes of a typical color element fade, this results in overall fading of the image over time. In addition, since the three image dyes may not fade at the same rate, an apparent change in image color may result. Such change is particularly noticable in the case of magenta image dye fading.
A variety of dye-forming coupler types have been used in photographic materials. Among the known dye-forming couplers are cyclic azoles such as pyrazolotriazoles, pyrazolobenzimidazoles, and imidazopyrazoles. These couplers contain bridgehead nitrogen 5,5 fused ring systems and include such couplers as pyrrolo[1,2-b]pyrazoles, pyrazolo[3,2-c][1,2,4]triazoles, pyrazolo[2,3-b][1,2,4]triazoles, imidazo[1,2-b]pyrazoles, imidazo[1,5-b]pyrazoles, imidazo[1,2-a]imidazoles, imidazo[1,2-b][1,2,4]tiiazoles, imidazo[2,1-c][1,2,4]tiiazoles, imidazo[5,1-c][1,2,4]triazoles and [1,2,4]triazolo[3,4-c][1,2,4]triazole. These couplers also contain bridgehead nitrogen 5,5,6 fused ring systems and include such as pyrazolo[3,2-b]benzimidazoles. These couplers may form magenta or cyan dyes, depending on the ring structure and substituents.
A significant disadvantage of pyrazoloazole couplers is fading of the dyes formed from them by photographic processing due to extended exposure to low levels of light. Compounds which are included in photographic elements to reduce image dye fading are lknown as stabilizers. Inclusion of stabilizers in color photographic materials can reduce the deterioration of the dye images which occurs over time as a result of the action of light, heat or humidity. This is true for dyes formed from pyrazoloazole couplers. U.S. Pat. Nos. 5,236,819 and 5,082,766 and German Published Patent Application DTOS 4,307,194 describe the use of certain stabilizers with pyrazoloazole couplers to improve their dye stability. However, it would be desirable to further improve the light stability of dyes derived from cyclic azole dye forming couplers, and thus retain the color rendition of the image for a longer period of time.